Fiber board sheets designated as the MDF (medium density fiber board) type are conventionally producing fiber in a defibrator, drying the fiber together with resin, wax, etc. in a tube drier, and transporting the dried fiber by means of a pneumatic or mechanical conveying system to a fiber bin, from which it is discharged in a controlled manner to a conveying system for feeding same to a mat-laying station, in which a fiber mat is formed in a controlled manner.
The thus-formed fiber mats are subsequently hot-pressed in the production line to form a complete MDF-sheet. Multi-layer sheets are also manufactured in a corresponding manner. In these processes a considerable amount of energy is employed from the drying step all the way to the forming step, for both the drying itself and in order to effect the conveying of the dried material. The moisture content of the fibers prior to the drying is about 100%, and after the drying is about 10%.
Because of the special requirements placed on the drying procedures required for the drying of fibers, it is not possible to use the more energy-efficient driers which are used, for example, in the particle board industry, but one is essentially limited to the use of a tube drier. This type of drier, however, requires a great amount of energy per ton of fiber. This energy is normally produced in a steam boiler, hot oil boiler, or the like, with the energy then being transferred to the drying gas by means of a heat exchanger.
Upon their discharge from the drier, the dried fibers generally have a temperature of between about 60 and 70 .degree. C. After being handled in the manner described above, the temperature of the fibers then drops to around room temperature. During the subsequent hot pressing the fiber mat must then be reheated to a temperature above about 100.degree. C. in order to bring about the intended curing of the resin which is required in order to obtain a sheet having sufficient strength. For these reasons, not only is additional energy lost, but at the same time the heating time in the hot press is rather long, and therefore the rate of production is restricted.
An object of the present invention is to reduce the energy demand during the manufacture of fiber board sheets while utilizing the dry method.